Group-III group-V compound semiconductors, commonly referred to as III-V compound semiconductors, have been under intense research in recent years due to their promising applications in electronic and optoelectronic devices. When the group V element in a III-V compound semiconductor is nitrogen, the compound semiconductor is referred to as a III-N compound semiconductor. The III-N compound semiconductor GaN is widely used in optoelectronic devices. Particular examples of potential optoelectronic devices employing GaN include blue light emitting diodes and laser diodes, and ultra-violet (UV) photo-detectors. The large bandgap and high electron saturation velocity of III-N compound semiconductors also make them excellent candidates for applications in high temperature and high-speed power electronics.
Due to the high equilibrium pressure of nitrogen at typical growth temperatures, it is extremely difficult to fabricate GaN bulk substrates. Owing to the lack of feasible bulk growth methods, films of GaN are commonly deposited epitaxially on dissimilar substrates, such as SiC or sapphire (Al2O3). However, a current problem with the manufacturing of GaN thin films is that there is no readily available suitable dissimilar substrate material whose lattice constant and thermal expansion coefficient closely matching those of GaN. If the difficulties of growing GaN films on silicon substrates could be overcome, silicon substrates would be attractive for GaN growth given their low cost, large diameter, high crystal and surface quality, controllable electrical conductivity, and high thermal conductivity. The use of silicon substrates would also provide easy integration of GaN based optoelectronic devices with silicon-based electronic devices.
Additionally, due to the lacking of appropriate substrates for growing GaN films thereon, the sizes of the GaN films are limited. The large stresses created by growing a GaN film on a dissimilar substrate may cause the substrate to bow. This bowing may cause several adverse effects. First, a great number of defects (dislocations) will be generated in the supposedly crystalline GaN films. Second, the thicknesses of the resulting GaN films will be less uniform, causing wavelength shifts of the light emitted by the optical devices formed on the GaN films. Third, cracks may be generated in large stressed GaN films.
Epitaxial lateral overgrowth (ELOG) techniques have been used to form GaN films on dissimilar substrates that have reduced stress and reduced dislocations therein. FIGS. 1 and 2 illustrate a structure comprising a film of GaN formed using ELOG. Referring to FIG. 1, underlayer 4, which comprises a III-N compound semiconductor, is formed on substrate 2, followed by the formation of III-N compound semiconductor layer 6. Recesses 7 are then formed in III-N compound semiconductor layer 6. Next, as shown in FIG. 2, first masks 8 and second masks 10 are formed on the upper surface portions of III-N compound semiconductor layer 6 and the bottoms of recesses 7, respectively. III-N compound semiconductor 12 is then epitaxially grown starting from the sidewalls of recesses 7. Having the III-N compound semiconductor 12 film grow laterally from the sidewalls tends to reduce the number of dislocations in the resulting film of III-N compound semiconductor 12. A disadvantage of the method shown in FIGS. 1 and 2 is that the formation of under-layer 4, III-N compound semiconductor layer 6, and masks 8 and 10 are additional process steps that increase the overall manufacturing cost.
FIGS. 3 and 4 illustrate an alternative ELOG process. First, as shown in FIG. 3, substrate 14 is provided. Recesses 15 are then formed in substrate 14. Next, as shown in FIG. 4, III-N compound semiconductor portions 16 are formed, which include portions 161 in recesses 15, and portions 162 on the protruding portions of substrate 14. The method shown in FIGS. 3 and 4 requires that the growth of III-N compound semiconductor films be suppressed on the sidewalls of recesses 15 so that portions 162 will grow laterally and coalesce with neighboring portions 162 in order to form a continuous film over substrate 14. However, the ELOG method shown in FIGS. 3 and 4 also has significant vertical growth of III-N compound semiconductor portions 162, so the reduction in the number of dislocations provided by lateral growth is limited. The process is further complicated by the fact that the recesses 15 must be deep enough to prevent 161 and 162 from coalescence. New methods that fully use the advantageous feature of ELOG, while at the same time having reduced process complexity, are thus needed.